Reading other answers I notice that they focus in the fact that ground wires have the same potential than earth (if they're properly grounded and maintained). But the op asks if it is dangerous to touch them. And I'll answer yes for two reasons:

1. A perfectly maintained ground wire is safe (most of the time), but ground wires are not always well maintained. In my experience they get little to no attention during inspections and because they do not show symptoms or malfunction when damaged as other parts of a system do, operators don’t notice when they get damaged. An interrupted ground wire is potentially more dangerous than a live wire, live wires have circuit breakers that are designed to cut the current flow when a failure occurs, ground wires don’t have circuit breakers, so when a failure occurs and the wire is energized noting will cut the current flow if the ground wire is damaged
2. When there’s a discharge (lighting strike for example) the ground system is designed to dissipate all the current to the ground, I know I am stretching the odds but when a lightning strikes a ground wire is the last thing I would wanna be touching.

So it is true that you most likely won’t get hurt if you touch a ground wire, but I’d advice you to don’t do it, it can be extremely dangerous given the wrong conditions.